1. Field of the Invention
This invention relates to a supercritical cycle, constituting a refrigeration cycle, using a refrigerant such as CO2 which, at high pressure, assumes a supercritical state. The invention also relates to an expansion valve used for the refrigeration cycle.
2. Description of the Related Art
In the conventional supercritical cycle, the opening degree of the expansion valve must be controlled to maximize the COP of the cycle with respect to the refrigerant temperature after the gas cooler. Such an expansion valve is described in Japanese Unexamined Patent Publication No. 2000-81157.
A known supercritical cycle including this expansion valve is shown in FIG. 11. The supercritical cycle 11 includes a compressor 13, a gas cooler 15, a temperature sensing portion 19 of an expansion valve 17, a main valve portion 21 of the expansion valve 17, an evaporator 23, an accumulator 25 and the compressor 13 arranged in that order so that the refrigerant is circulated in the same order. Also, an internal heat exchanger 27 is arranged in the refrigerant path between the accumulator 25 and the compressor 13 and in the refrigerant path between the temperature sensing portion 19 of the expansion valve 17 and the main valve portion 21. The heat is moved from the high-pressure refrigerant downstream of the gas cooler 15 to the low-pressure refrigerant downstream of the accumulator 25 and, thus, the enthalpy of the refrigerant at the inlet of the evaporator 23 is reduced thereby to improve the refrigeration capacity of the CO2 cycle.
In this supercritical cycle 11, the refrigerant temperature at the outlet of the gas cooler 15 is detected by the temperature sensing portion 19 and, therefore, the refrigerant at the outlet of the gas cooler 15, after being supplied to the temperature sensing portion 19 of the expansion valve 17, is required to be returned to the inlet of the main valve portion 21 of the expansion valve 17 again through the internal heat exchanger 27. Although the gas cooler 15, the expansion valve 17 and the evaporator 23 can be continuously coupled, the internal heat exchanger 27 is arranged in U turn in which the refrigerant flows from the temperature sensing portion 19 and returns to the main valve portion 21 through the internal heat exchanger 27. As a result, a large space is required around the expansion valve 17, thereby posing the problem that the expansion valve cannot be easily arranged in a small engine compartment.
Also, the internal heat exchanger 27 is arranged in U turn with respect to the expansion valve 17 and cannot be arranged between the devices, thereby requiring extra piping. Especially, the internal heat exchanger 27 in a double-pipe structure, which could be used as a part of the piping system if arranged between the devices, cannot be effectively used in the U-turn arrangement.
Further, in view of the fact that the inlet and the outlet of the internal heat exchanger 27 are connected to the expansion valve 17, four joints are required for connection to the expansion valve 17. A problem results in that not only the cost is increased but also the expansion valve becomes bulky.
Furthermore, the expansion valve is controlled in such a manner that the temperature of the inflowing refrigerant is detected by the CO2 gas sealed in the temperature sensing portion therein and the high pressure is controlled to a maximum COP. The CO2 gas has a low critical temperature of 31° C. and, in the case where the atmospheric temperature is high, therefore, the CO2 gas sealed in the temperature sensing portion assumes a supercritical state.
With the increase in the refrigerant temperature at the outlet of the gas cooler, i.e. the temperature of the refrigerant flowing into the temperature sensing portion of the expansion valve, therefore, the control pressure of the expansion valve is also undesirably increased. Especially in the case where the intake air temperature of the gas cooler is high such as during the idling, for example, the refrigerant temperature at the outlet of the gas cooler increases to such an extent that the control pressure reaches the upper limit of the high pressure. In order to suppress the increase in the high pressure, therefore, the compressor capacity is required to be decreased, thereby posing the problem that the cooling capacity is considerably reduced. In the case where the pressure further increases to a still higher abnormal level, the compressor may be stopped.